Horizontal sync signal generating device for use in optical printer

ABSTRACT

A horizontal sync signal generating device for use with an optical printer to determine the starting positions of horizontal scannings on a photoconductive member for forming images thereon. The generation of horizontal sync timing signals is kept constant regardless of any difference in the characteristics of a light beam transmitted via each scanning face of a polygonal rotary member.

BACKGROUND OF THE INVENTION

This invention relates to an optical printer, such as a laser beamprinter, and more particularly to a horizontal sync signal generatingdevice for synchronizing the horizontal scanning starting positions ofan optical beam, such as a laser beam.

The fundamental structure of the laser beam printer (hereinafterreferred to "LBP") is as illustrated in FIG. 6, wherein a laser beamprojected from a semiconductor laser 11 and containing image data, suchas graphics and characters, is directed to a rotatably driven polygonalmirror 12 via a collimator lens 16 and a cylindrical lens 17 and isreflected on its reflection planes 12a through 12f. The reflections areradiated onto a photoconductive drum 13. Located between the polygonalrotary mirror 12 and the photoconductive drum 13 is a fθ lens 14 whichmakes constant a horizontal scanning speed of the laser beam on the drum13. While horizontal scannings are carried out by the reflection planesof the polygonal mirror 12, the photoconductive drum 13 is rotated at apredetermined speed in a vertical scanning direction perpendicular tothe horizontal scanning direction. Thus, latent images of entire videosignals are formed on the photoconductive drum 13. The latent images arethen developed on paper using conventional copy machine technology.

In the LBP as described above, an individual horizontal scanning startposition, that is, printing start point of the latent image formed onthe photoconductive drum 13, must be synchronized to be brought intoalignment with one another in the vertical scanning direction. For thispurpose, a photodetector 15 is located on one side of the drum 13 wherethe laser beam is admitted with rotation of the polygonal mirror 12without affecting formation of the latent images. Horizontalsynchronization can be obtained by setting the scanning start positionson the drum 13 based upon the incidence of the laser beam onto thephotodetector 15.

FIG. 7 shows a conventional horizontal sync signal generating deviceusing the above photodetector 15. When the beam enters the photodetector15 (which comprises a photodiode PD), the electric current ip whichdepends on the quantity of the received light that flows throughtransistor TR1 and load resistor RL. The voltage across the loadresistor RL is compared with a reference voltage Vref in a comparator C.A horizontal sync signal HL (pulse) is then outputted when the voltageacross the load resistor RL exceeds the reference voltage Vref.

Assuming that the reference voltage Vref is constant, timing thegeneration of the horizontal sync signal is dependent on the magnitudeof the voltage across the load resistance RL, that is; the quantity oflight admitted into the photodiode PD. Voltage Vx across the loadresistance RL is given by the equation : Vx=ip×hFE (amplification rateof the transistor TR1) x RL (resistance value of the load resistance)RL.

FIG. 8 which shows the timing the generation of the horizontal syncsignal HL in relation with the voltage across the load resistor RL,reveals that the smaller the voltage across the load resistor RL, themore delayed is the generation of the horizontal sync signal HL. Thedelay time is indicated as .increment.t. This means that, ifreflectivity is different among the reflection planes 12a to 12f of therotary polygon mirror 12, the scanning start positions formed on thephotoconductive drum 13 differ at every horizontal scanning and are outof alignment in the vertical scanning direction. In fact, it isdifficult to keep the reflectivity on the reflection planes 12a to 12fconstant. The difference in reflectivity tends to increase as theoperation goes on because of aging, deposition of dust, and etc. Suchmisalignment in print starting positions results in a poor printingquality, including non-straight vertical lines, which are particularlyundersirable when printing graphics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedhorizontal sync signal generating device for use in an optical printerwherein the print starting position can be made consistent even if thereflectivities are different among reflection planes of a rotary polygonmirror.

The foregoing object is accomplished by a device wherein the timing ofgenerating horizontal sync signals is kept constant regardless ofdiffering reflectivity on the reflection planes by varying a referencevoltage inputted to a comparator.

More particularly, according to the invention, there is provided ahorizontal sync signal generating device for use in an optical printerwhich comprises a photodetector for receiving light beams containingimage information transmitted via a polygonal rotary member having aplurality of scanning faces, and a photoconductive member on whichlatent images are formed by said light beams. Starting positions ofhorizontal scannings on the photoconductive member for forming thelatent images thereon are determined by the timing of incident lightbeams onto the photodetector. The improvement comprises:

means for detecting at least one characteristic of the said light beamtransmitted via each scanning face of the polygonal rotary member,

means for setting an associated voltage value that is dependent on thecharacteristic of the light beam detected by the charcteristic detectingmeans, and

means for comparing the associated voltage value set for each scanningface by the associated voltage setting means with a voltage value thatis generated by the photodetector and for generating a horizontal syncsignal when the voltage value exceeds the associated voltage.

With the above constructed device, the timing at which a horizontal syncsignal is generated can be kept constant in line with the verticalscanning direction, regardless of different reflectivities on thereflection planes of the polygon mirror.

Furthermore, false horizontal sync signals, which may be generatedbecause of light noise such as ghosts, can be eliminated by adding meansfor supplying a blocking to the comparing means at predeterminedintervals, the blocking voltage supplying means comprising means formaintaining the blocking voltage at a level exceeding a possible maximumvoltage output from the photodetector upon receipt of the light beams,thereby precluding irregularities in print starting positions.

These and other objects, features and advantages of the presentinvention may be readily understood with reference to the followingdescriptions of preferred embodiments of the invention, taken togetherwith the accompanying drawings.

DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 shows a circuit diagram of a horizontal sync signal generatingdevice of a laser beam printer embodying the invention;

FIG. 2 is a timing chart illustrating an example of operation in thedevice shown in FIG. 1;

FIG. 3 is a graph of a Gaussian distrubution indicating a peak value ofa light beam and the magnitude of an associated voltage determined independence on the peak value;

FIG. 4 is a flow chart showing a subroutine for storing the peak valueof each reflection plane of a polygonal mirror;

FIG. 5 is a timing chart showing an optional feature of the invention;

FIG. 6 shows a structural view of a basic optical system of a laser beamprinter;

FIG. 7 shows a circuit diagram of a prior art horizontal sync signalgenerating device; and

FIG. 8 is a graph showing a deviation between the magnitude ofreflections and the timing for generating horizontal sync signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the invention are now described withreference to the accompanying drawings.

FIG. 1 shows a diagramatic view of the horizontal sync signal generatingdevice for the LBP embodying the invention, wherein a photodiode PD anda comparator C are identical with those used in the prior art deviceillustrated in FIG. 7. In this device, however, the output of thephotodiode PD is connected to a peak hold circuit 21 and a secondcomparator 25, as well as the comparator C.

The peak hold circuit 21 comprises a capacitor 21c charged to a peakvalue Pv which corresponds to a maximum quantity of light entered ontothe photodiode PD, a diode 21b for preventing the counter flow of theelectric current and a resistor 21d, at the input and output sides ofwhich a pair of buffers 21a and 21e are arranged.

The output of the peak hold circuit 21 is connected to a CPU (CentralProcessing Unit) 23 via an A/D converter 22 so that the obtained peakvalue Pv is fed to the CPU 23 after being converted to a correspondingdigital signal by the A/D converter 22. The CPU 23 stores the digitalinformation corresponding to the peak value Pv in a RAM (Random AccessMemory) 24. The output of the capacitor 21c in the peak hold circuit 21is also connected to a transistor TR2 so that, when the transistor TR2is turned on, the capacitor 21c is discharged so as to make it possibleto charge and retain the peak value Pv of the next beam which isreflected on the next reflection plane of the polygonal mirror 12.

The output of the photodiode PD is fed to the second comparator 25 thatis used for generating an output signal when the voltage level inputtedinto the second comparator 25 is higher than a predetermined levelvoltage. The generated signal is fed to the CPU 23, which is recognizedas an interruption signal INT by the CPU 23. The predetermined voltagelevel to be compared with the input level has been set to be lower thanthe possible level of the output of the photodiode PD generated when thephotodiode PD receives the reflected beam from the polygonal mirror 12.

When the interruption signal INT is received from the second comparator25, the CPU 23 interrupts the routine processings for the LBP andexecutes the subroutine shown in FIG. 4, which will be explained laterto store a peak value Pv1-Pv6 detected by the peak hold circuit 21 inthe RAM 24 for each of reflection planes 12a through 12b of the rotarypolygonal mirror 12.

The comparator C generates the horizontal sync signals, as the prior artdevice illustrated in FIG. 7. An associated voltage Vcmp is fed to thecomparator C which is not a predetermined voltage, but rather variesdepending upon the reflection rate of each reflection plane 12a through12f of the polygonal mirror 12. In other words, the comparator C isconnected to the CPU 23 via a D/A converter 26 and the associatedvoltage Vcmp is varied based upon the signals fed from the CPU 23through the D/A converter 26.

The operation of the above constructed horizontal sync signal generatingdevice is explained in conjunction with FIGS. 2 through 4.

The above device can be operated in a single rotation of the rotarypolygonal mirror 12 before starting the main operation of the LBP. Thetiming for operating the device can be arbitrarily selected, so long asthe polygonal mirror 12 is rotating and the semiconductor laser 11 isstably projecting the laser beam, at such time as immediately beforestarting the printing by the LBP.

FIG. 2 is a timing chart illustrating the steps to operate the abovedevice. When a motor, not shown, for polygonal the rotary mirror 12 isturned on, the motor rpm gradually increases and reaches its stabilizedstate in about 10 seconds. This length of time is utilized to measurethe reflectivity on each reflection plane 12a through 12f of thepolygonal mirror 12. That is, when the polygonal mirror 12 begins torotate, photodiode PD receives the reflected beam from any one of thereflection planes 12a to 12f of the polygonal mirror 12, regardless ofthe rotary speed of the mirror 12. The voltage corresponding to thisreflected beam is fed to the peak hold circuit 21, as well as to thesecond comparator 25. The output from the second comparator 25 causesthe interruption in the CPU 23.

Upon receipt of the interruption signal INT from the second comparator25, the CPU 23 executes the subroutine illustrated in FIG. 4 for storingin the RAM 24 the peak value Pv1-Pv6 held in the capacitor 21c of thepeak hold circuit 21. That is, the CPU 23 orders the A/D converter 22 toinput the voltage (peak value Pv1) charged in the capacitor 21c and toconvert it to a digital representation. This order is fed to the A/Dconverter 22 with a certain delay from the receipt of the interruptionsignal INT for taking a time during which the capacitor 21c accuratelycharges the voltage corresponding to the maximum quantity of the lightentered into the photodiode PD. After confirming the completion of theA/D convertion in the A/D converter 22, the CPU 23 stores the converteddata of the peak value Pv1 in the address 1 of the RAM 24 (see FIG. 2).When the peak value Pv1 for one of the reflection planes 12a to 12f isstored, the CPU 23 feeds a RESET signal to turn on the transistor TR2.This causes the capacitor 21c of the peak hold circuit 21 to bedischarged so that the circuit 21 is ready for storing the peak valuePv2 of the next reflection plane.

The above steps are repeatedly executed upon receipt of the interruptionsignal INT until a hexadecimal counter, not shown, counts up to six (6)so that all the peak values Pv1-Pv6 for the reflection planes 12athrough 12f are all stored in the specified addresses within the RAM 24.However, once the peak value Pv1-Pv6 for each of reflection planes 12athrough 12f is completely stored, the CPU 23 ignores the input signalfrom the second comparator 25 as the interruption signal INT bututilizes it as a timing signal for calculating a data to be fed to theD/A converter 26 to control the associated voltage Vcmp1-Vcmp6 in thecomparator C.

The calculation by the CPU 23 is accomplished, in this embodiment, bymultiplying the peak value Pv by 1/2, since the laser radiation curveapproximately forms a Gaussian distribution. In FIG. 3, whichillustrates Gaussian distribution curves, assuming the result ofmultiplying the peak value Pv by 1/2 as the associated voltage Vcmp, thehorizontal timing HLt at which the voltage Vcmp crosses the Gaussiancurve (that is, the timing at which the horizontal sync signal HL isgenerated) is kept constant regardless of the level of the peak valuePv. This relationship applies to any case where the peak value Pv isdivided by any integer, except for the top and bottom area of theGaussian curve. That is, dividing by 2 is particularly chosen because itenables the use of the middle area of the Gaussian curve, where thecurve section approximates linearity and also allows such simpleoperation as mere right shift (one bit) on a computer. If the lightreceiving area of the photodiode PD used as the photodetector is largerelative to the laser beam diameter, photodiode PD is able to produce anoutput approximating the Gaussian distribution by providing a slit orpinhole on the light receiving area thereof.

Once the peak values Pv1-Pv6 for reflection planes 12a to 12f are storedin the RAM 24, the CPU 23 fetches the peak value Pv1-Pv6 every time itreceives the output signal from the second comparator 25 with furtherrotation of the polygon mirror 12 and shifts it right one bit andmultiplies by 1/2, the result being fed to the D/A converter 26 togenerate the individual associated voltage Vcmp1-Vcmp6 suitable for eachreflection plane 12a through 12f of the polygonal mirror 12 on which thelaser beam received by the photodiode PD is reflected.

The associated voltage Vcmp1-Vcmp6 fed to the comparator C by the D/Aconverter 26 is thus made dependent on the differing reflectivity of thereflection planes 12a through 12f of the polygonal mirror 12 so that thetiming at which the horizontal sync signal HL is generated by thecomparator C is kept constant throughout every horizontal scannings.Consequently, the start points of horizontal scannings on thephotoconductor drum 13 are in alignment with one another in the verticalscanning direction, with no irregularity in beginning characters nordistorted vertical lines.

The peak value Pv1-Pv6 mentioned above stored in the RAM 24, may bereplaced by digital information corresponding to the associated voltageVcmp1-Vcmp6 after calculation; that is, the digital value obtained byshifting to the right on the computer.

Although in the described embodiment, the associated voltage Vcmp1-Vcmp6is maintained at the value set by the D/A converter 26 between thesuccessive receipts of the laser beams by the photodiode PD, there issome possibility that some irregular operation may occur in the device.That is, in the LBP optical system, reflections of the light maysometimes cause an unexpected light noise such as a ghost. Such lightnoise, when entering the photodetector 15 (photodiode PD), is likely togenerate an unnecessary horizontal sync signal HL. Horizontal syncsignals other than those properly generated creates irregularities inthe horizontal scanning starting position on the photosensitive drum 13.

Accordingly, as illustrated in FIG. 5, the associated voltageVcmp1-Vcmp6, calculated in the CPU 23, may be fed to the D/A converter26 and then to the comparator C at a specified timing shortly before thetiming for generating the horizontal sync signal HL, and a blockingvoltage Vbck may be supplied between the successive receipts of thelaser beams by the photodiode PD. The blocking voltage Vbck shall bemaintained at a level exceeding a possible maximum voltage output fromthe photodiode PD upon recipt of the light beams. The above specifiedtiming can be taken by the timing data stored in the RAM 24, which areobtained by calculation based upon factors such as the rotary speed ofpolygonal mirror 12 under the stable conditions, the length of thehorizontal scanning on the drum 13 and so on. Alternatively, the timingdata may be prepared by the CPU 23 by detecting and analyzing the actualdata during the operations of the LBP. Such timing for feeding theassociated voltage Vcmp1-Vcmp6 to the D/A converter 25 and thus thecomparator C can be easily set by the CPU 23.

With this optional arrangement, the associated voltage Vcmp1-Vcmp6 isfed to the comparator C shortly before the horizontal sync signal HL isgenerated. Thus, all the preceding signals given to the photodetector 15(photodiode PD) due to light noise are ignored.

Although, in the foregoing embodiment, the horizontal scanning iscarried out by the polygonal rotary mirror, the principles of thepresent invention can be applied to other scanning systems, such as aholographic system.

What is claimed is:
 1. A horizontal sync signal generating device,comprising:a photodetector for receiving light beams containing imageinformation which are transmitted by a rotatable polygonal member onwhich latent images are adapted to be formed by said light beams; meansfor detecting a peak voltage corresponding to a maximum quantity oflight entered onto said photodetector for detecting at least onecharacteristic representative of each of said light beams transmittedvia each scanning face of said rotatable polygonal member; means forsetting an associated voltage that is dependent upon said characteristicof each of said light beams which is detected by said peak voltagedetecting means; means for comparing said associated voltage set foreach of said scanning faces by said associated voltage setting meanswith a voltage value generated by said photodetector and for generatinga horizontal sync signal when said voltage attains a predeterminedrelationship with respect to said associated voltage; and means forselectively preventing said comparing means from generating any outputsignal by blocking the generation of an undesired horizontal sync signalunless said voltage attains said predetermined relationship with respectto said associated voltage during a prescribed period of time.
 2. Adevice in accordance with claim 1, wherein said means for selectivelypreventing said comparing means from generating any output signalcomprises supplying means for supplying a blocking voltage to saidcomparing means at predetermined intervals, and for maintaining saidblocking voltage at a level which exceeds a maximum potential voltageoutput from said photodetector upon receipt of said light beams.
 3. Adevice in accordance with claim 1, said comparing means comprising meansfor generating a horizontal sync signal when said voltage generated bysaid photodetector exceeds said associated voltage.
 4. A horizontal syncsignal generating device comprising:a photodetector for receiving lightbeams containing image information which are transmitted by a rotatablepolygonal member having a plurality of scanning faces; a peak holdcircuit for detecting at least one characteristic of each of said lightbeams which are transmitted via each scanning face of said rotatablepolygonal member; subjecting each characteristic to a Gaussiandistribution so as to obtain a reliable reference voltage for eachscanning face of said rotatable polygonal member; means for comparingsaid reference voltage with a voltage generated by said photodetector,generating a horizontal sync signal when said voltage generated by saidphotodetector attains a predetermined relationship with respect to saidreference voltage; and means for supplying a blocking voltage to saidcomparing means at predetermined intervals and for maintaining saidblocking voltage at a level which exceeds a maximum potential voltageoutput from said photodetector upon receipt of said light beams so thatsaid horizontal sync signal can only be generated when saidpredetermined relationship exists when said blocking voltage is notsupplied to said comparing means.
 5. The device of claim 4, saidcomparing means comprising means for generating a horizontal sync signalwhen said voltage generated by said photodetector exceeds said referencevoltage.